While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration densities and operating speeds in LSI devices, DUV and EUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser is requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. While the ArF immersion lithography has entered the commercial stage, the technology still needs a resist material which is substantially non-leachable in water.
In the ArF lithography (193 nm), a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polyacrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymerization (ROMP) polymers, and hydrogenated ROMP polymers have been proposed as the base resin. This choice is effective to some extent in enhancing the transparency of a resin alone.
With the rapid progress toward miniaturization, it becomes difficult to form a pattern of desired size from such a resist material. In particular, the influence of acid diffusion is detrimental to lithography performance. As the pattern size is approaching the diffusion length of acid, the degradation of contrast becomes more serious. As the mask error factor (MEF), indicative of a dimensional shift on wafer relative to a dimensional shift on mask, increases, a noticeable drop of mask fidelity ensues. In addition, since the fluctuation of pattern line width, known as line width roughness (LWR), and the critical dimension uniformity (CDU) of patterns are largely affected by acid diffusion, degradation of these parameters becomes a problem.
To solve the problems, studies have been made on acid diffusion inhibitors as well as base resins and photoacid generators. Amines are typically used as the acid diffusion inhibitor. Many problems associated with LWR and CDU as an index of pattern roughness are left unsolved. Also the use of weak acid onium salts as the diffusion inhibitor is under study. For example, Patent Document 1 describes a positive photosensitive composition for ArF excimer laser lithography comprising a carboxylic acid onium salt. This system is based on the mechanism that a salt exchange occurs between the weak acid onium salt and a strong acid (sulfonic acid) generated by a PAG upon exposure. That is, the strong acid (α,α-difluorosulfonic acid) having high acidity is replaced by a weak acid (alkanesulfonic acid or carboxylic acid), thereby suppressing acidolysis reaction of acid labile group and reducing or controlling the distance of acid diffusion. The onium salt apparently functions as a quencher, that is, acid diffusion inhibitor. However, as the microfabrication technology is currently further advanced, the resist composition using such weak acid onium salt becomes unsatisfactory in lithography performance, particularly when processed by the ArF immersion lithography.